How do we get from organic interstellar molecules to the amino acids necessary for the development of life? One element of the answer was provided by the decisive experiment carried out in 1953 by Stanley Miller and Harold Urey. Several decades earlier, Oparin had suggested that micro-organisms could appear, in a reducing medium, at the end of a long series of chemical reactions involving complex organic molecules. Miller and Urey undertook to simulate the atmosphere of the primordial Earth by means of a reducing medium, based on hydrogen, methane, and ammonia, in the presence of liquid water, and repeatedly subject to electrical discharges
Fig. 9.7 The experiment by Miller and Urey, used to carry out abiotic synthesis of amino acids. The lower bulb, heated to boiling point, simulates a liquid ocean. The water vapour enters the upper bulb which simulates a reducing atmosphere based on hydrogen, methane and ammonia. The energy source is provided by electrical discharges
(Fig. 9.7). After a week, numerous complex organic molecules had formed, including amino acids.
Since Miller and Urey's historical experiment, numerous simulations have been carried out in the laboratory, with different sources of energy and different initial mixtures. All led to the formation of molecules that were biologically significant, provided that the initial mixture was reducing. Here a difficulty appears: we have seen (Chap. 3) that the Earth's primitive atmosphere, like that of Venus and Mars, was definitely not reducing, but was dominated by carbon dioxide and nitrogen. How can we explain the appearance of the first prebiotic molecules on Earth? Perhaps we need to envisage an external source, by way of meteoritic bombardment.
Assuming an external source of amino acids, did the primitive Earth have sources of energy that could drive the chemical reactions that would evolve to life? Among the possible sources (sunlight, electrical discharges from thunderstorms, shockwaves from meteoritic impacts, etc.), solar ultraviolet radiation is by far the predominant source. Stellar evolution models suggest that solar radiation would have been less than its current value by about 20-30 percent, but the energy contribution of solar UV radiation to the primitive Earth was still more than one thousand times that of the other sources of energy just mentioned.
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